1. Field of the Invention
The present invention relates to a semiconductor laser device including an ARROW (Antiresonant Reflecting Optical Waveguide) structure. The present invention also relates to a process for producing a semiconductor laser device including an ARROW structure.
2. Description of the Related Art
A reliable high-power semiconductor laser device which emits a high-quality, diffraction-limited beam is required for use as a light source for exciting an optical fiber amplifier.
U.S. Pat. No. 5,606,570 discloses a semiconductor laser device having an ARROW (Antiresonant Reflecting Optical Waveguide) structure as a semiconductor laser device which can emit a high-power, diffraction-limited laser beam in the 980 nm band. The ARROW structure is a structure for confining light in a core region. The disclosed ARROW structure includes a plurality of core regions having a low equivalent (effective) refractive index, high-refractive-index regions which have a high equivalent refractive index and are arranged between the plurality of core regions and on the outer sides of the plurality of core regions, and low-refractive-index regions which have a low equivalent refractive index and are arranged on the outer sides of the outermost ones of the high-refractive-index regions. The high-refractive-index regions function as a reflector of light in the fundamental mode, and the low-refractive-index regions suppress leakage of light. Thus, the semiconductor laser device can be controlled so as to operate in the fundamental transverse mode.
In addition, it is reported that a preferable value of the width db1xe2x80x2 of each of the outermost ones of the high-refractive-index regions is determined in accordance with the equation (1), a preferable value of the width db2xe2x80x2 of each of the high-refractive-index regions arranged between the plurality of core regions is determined in accordance with the equation (2), and a preferable value of the width of each of the low-refractive-index regions is dcxe2x80x2/2, where dcxe2x80x2 is the width of each of the plurality of core regions. In the equations (1) and (2), xcex is the oscillation wavelength, ncxe2x80x2 is the equivalent refractive index of the plurality of core regions, and nbxe2x80x2 is the equivalent refractive index of the high-refractive-index regions.                               d          b1          xe2x80x2                =                              3            ⁢                          xe2x80x83                        ⁢            λ                                4            ⁢                                          {                                                      n                    b                    xe2x80x22                                    -                                      n                    c                    xe2x80x22                                    +                                                            (                                              λ                                                  2                          ⁢                                                      d                            c                            xe2x80x2                                                                                              )                                        2                                                  }                                            1                2                                                                        (        1        )                                          d          b2          xe2x80x2                =                                            xe2x80x83                        ⁢            λ                                2            ⁢                                          {                                                      n                    b                    xe2x80x22                                    -                                      n                    c                    xe2x80x22                                    +                                                            (                                              λ                                                  2                          ⁢                                                      d                            c                            xe2x80x2                                                                                              )                                        2                                                  }                                            1                2                                                                        (        2        )            
However, the semiconductor laser device disclosed in U.S. Pat. No. 5,606,570 includes an active layer made of InGaAs, and the ARROW structure is formed with a current confinement layer made of InGaAlP and a high-refractive-index region made of GaAs by using a regrowth technique. In addition, GaAs and InGaP layers (or InAlP, GaAs, and InGaP layers) are exposed at the base surface on which a cladding layer is regrown. Therefore, Pxe2x80x94As interdiffusion occurs at the exposed surface during a process of raising temperature for the regrowth, and thus the regrowth is likely to become defective. As a result, the above semiconductor laser device is not actually used. Further, there is a high degree of technical difficultly in regrowing layers when a layer (such as the InAlP layer) exposed at the base surface on which the cladding layer is regrown contains aluminum, which is prone to oxidation.
An object of the present invention is to provide a semiconductor laser device which includes an ARROW structure and is not technically difficult to produce.
Another object of the present invention is to provide a process for producing with high precision a reliable semiconductor laser device which includes an ARROW structure.
(I) According to the first aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a first lower cladding layer formed above the GaAs substrate and made of In0.49Ga0.51P of the first conductive type; a lower optical waveguide layer formed above the first lower cladding layer and made of Inx1Ga1-x1As1-y1Py1 which is undoped or the first conductive type, where x1=0.49y1 and 0xe2x89xa6y1xe2x89xa60.3; a compressive-strain quantum-well active layer formed above the lower optical waveguide layer and made of Inx3Ga1-x3As1-y3Py3 where 0.49y3 less than x3 xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1; an upper optical waveguide layer formed above the compressive-strain quantum-well active layer and made of Inx1Ga1-x1As1-y3Py1 which is undoped or a second conductive type, where x1=0.49y1 and 0xe2x89xa6y1xe2x89xa60.3; a first upper cladding layer of the second conductive type, formed above the upper optical waveguide layer and made of one of In0.49Ga0.51P and AlxGa1-xAs which has an approximately identical refractive index to a refractive index of In0.49Ga0.51P, where 0.45xe2x89xa6xxe2x89xa60.55; a first etching stop layer formed above the first upper cladding layer and made of GaAs of the second conductive type; a second etching stop layer made of Inx8Ga1-x8P of the second conductive type and formed above the first etching stop layer other than stripe areas of the first etching stop layer corresponding to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval, where 0xe2x89xa6x8xe2x89xa61, and the stripe areas of the first etching stop layer extend in a direction of a laser resonator; a first current confinement layer made of GaAs of the first conductive type and formed above the second etching stop layer; a third etching stop layer made of Inx9Ga1-x9P of the second conductive type and formed over the first current confinement layer and the stripe areas of the first etching stop layer, where 0xe2x89xa6x9xe2x89xa61; a fourth etching stop layer made of GaAs of the second conductive type and formed above the third etching stop layer other than at least one stripe area of the third etching stop layer corresponding to the at least one current injection region; a second current confinement layer made of In0.49Ga0.51P of the first conductive type and formed above the fourth etching stop layer; a second upper cladding layer of the second conductive type, formed above the second current confinement layer and the at least one stripe area of the third etching stop layer, and made of one of In0.49Ga0.51P and AlxGa1-xAs which has an approximately identical refractive index to the refractive index of In0.49Ga0.51P, where 0.45xe2x89xa6xxe2x89xa60.55; and a contact layer made of GaAs of the second conductive type and formed above the second upper cladding layer.
(i) The current injection region may have a width equal to or greater than 3 micrometers.
(ii) The first current confinement layer may include first and second sublayers made of GaAs of the first conductive type, and a quantum-well layer formed between the first and second sublayers and made of an InGaAs material which has a smaller bandgap than the bandgap of the compressive-strain quantum-well active layer.
(iii) The semiconductor laser device according to the first aspect of the present invention may further comprise a second lower cladding layer made of AlxGa1-xAs of the first conductive type and formed between the GaAs substrate and the first lower cladding layer, where 0.45xe2x89xa6xxe2x89xa60.55. In this case, it is preferable that the thickness of the InGaP lower cladding layer does not exceed 0.5 micrometers.
(II) According to the second aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming above a GaAs substrate of a first conductive type a lower cladding layer made of In0.49Ga0.51P of the first conductive type; (b) forming above the lower cladding layer a lower optical waveguide layer made of In0.49Ga0.51As1-y1Py1 which is undoped or the first conductive type, where x1=0.49y1 and 0xe2x89xa6y1xe2x89xa60.3; (c) forming above the lower optical waveguide layer a compressive-strain quantum-well active layer made of Inx3Ga1-x3As1-y3Py3, where 0.49y3 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1; (d) forming above the compressive-strain quantum-well active layer an upper optical waveguide layer made of Inx1Ga1-x1As1-y1Py1 which is undoped or a second conductive type, where x1=0.49y1 and 0xe2x89xa6y1xe2x89xa60.3; (e) forming above the upper optical waveguide layer a first upper cladding layer of the second conductive type which is made of one of In0.49Ga0.51P and AlxGa1-xAs having an approximately identical refractive index to a refractive index of In0.49Ga0.51P, where 0.45xe2x89xa6xxe2x89xa60.55; (f) forming above the first upper cladding layer a first etching stop layer made of GaAs of the second conductive type; (g) forming above the first etching stop layer a second etching stop layer made of Inx8Ga1-x8P of the second conductive type, where 0xe2x89xa6x8xe2x89xa61; (h) forming above the second etching stop layer a first current confinement layer made of GaAs of the first conductive type; (i) removing stripe regions of the first current confinement layer and the first etching stop layer which extend in a resonator direction so as to produce an intermediate layered structure in which stripe areas of the first etching stop layer are exposed, where the stripe areas of the first etching stop layer extend in a direction of a laser resonator, and correspond to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval; (j) raising the temperature of the intermediate layered structure in an arsenic atmosphere; (k) forming above the first current confinement layer and the stripe areas of the first etching stop layer a third etching stop layer made of Inx9Ga1-x9P of the second conductive type, where 0xe2x89xa6x9xe2x89xa61; (1) forming above the third etching stop layer a fourth etching stop layer made of GaAs of the second conductive type; (m) forming above the fourth etching stop layer a second current confinement layer made of In0.49Ga0.51P of the first conductive type; (n) removing stripe regions of the second current confinement layer and the fourth etching stop layer so that at least one stripe area of the third etching stop layer is exposed, where the at least one stripe area of the third etching stop layer corresponds to the at least one current injection region; (o) forming above the second current confinement layer and the at least one stripe area of the third etching stop layer a second upper cladding layer of the second conductive type which is made of one of In0.49Ga0.51P and AlxGa1-xAs having an approximately identical refractive index to the refractive index of In0.49Ga0.51P, where 0.45xe2x89xa6xxe2x89xa60.55; and (p) forming above the second upper cladding layer a contact layer made of GaAs of the second conductive type.
(III) The semiconductor laser device according to the first aspect of the present invention and the process according to the second aspect of the present invention have the following advantages.
(i) In the semiconductor laser device according to the first aspect of the present invention, the first current confinement layer made of GaAs of the first conductive type has a higher refractive index than the refractive indexes of the second current confinement layer made of In0.49Ga0.51P of the first conductive type and the second upper cladding layer of the second conductive type. Therefore, in the active layer, first high-refractive-index regions which have a high equivalent refractive index are realized between the at least one core region (corresponding to the at least one current injection region each of which has a stripe shape) and on the outer sides of the at least one core region in the direction perpendicular to thickness and propagation of light, low-refractive-index regions which have a low equivalent refractive index are realized on the outer sides of the outermost ones of the first high-refractive-index regions in the direction, and second high-refractive-index regions which have a high equivalent refractive index are realized on the outer sides of the low-refractive-index regions in the direction. That is, the distribution of the equivalent refractive index of the active layer in the direction perpendicular to thickness and propagation of light realizes the aforementioned ARROW structure.
Since the semiconductor laser device according to the first aspect of the present invention includes the ARROW structure, the semiconductor laser device according to the first aspect of the present invention can emit a single peak beam in a transverse mode which is more effectively controlled than that in semiconductor laser devices which do not include the ARROW structure, even when the stripe width is increased.
In order to effectively control the transverse mode oscillation in the semiconductor laser devices which do not include the ARROW structure, the stripe width is required to be reduced to 3 micrometers or smaller, i.e., the width of the active region is required to be reduced. Therefore, when the output power is increased, the optical density in the active layer increases, and thus facet degradation is likely to occur. Consequently, the semiconductor laser devices which do not include the ARROW structure cannot operate with high output power in an effectively controlled transverse mode.
On the other hand, since the semiconductor laser device according to the first aspect of the present invention includes the ARROW structure, light can be effectively confined in a wide stripe (active) region in the semiconductor laser device according to the first aspect of the present invention, and therefore the semiconductor laser device according to the first aspect of the present invention can emit laser light in the fundamental transverse mode from the wide active region. In particular, when the width of the active region is increased to 3 micrometers or greater, the optical density in the active layer can be reduced, and therefore the temperature rise due to the non-radiative recombination in the vicinity of the end facet can be suppressed. Thus, the semiconductor laser device according to the first aspect of the present invention can emit a laser beam in the fundamental transverse mode with higher power than the semiconductor laser devices which do not include the ARROW structure.
(ii) When the process according to the second aspect of the present invention is used, a semiconductor laser device including an ARROW structure, which is realized with the aforementioned first and second high-refractive-index regions and low-refractive-index regions in the active layer, can be easily produced with high precision.
(iii) According to the construction of the semiconductor laser device according to the first aspect of the present invention, which can be produced by the process according to the second aspect of the present invention, the layers exposed at the base surface on which the third etching stop layer is regrown are only GaAs layers, and the layers exposed at the base surface on which the second upper cladding layer is regrown are only InGaP layers, i.e., the base surfaces of regrowth do not contain aluminum. Therefore, the regrowth of the second upper cladding layer does not have technical difficulty.
(iv) In addition, since As and P do not concurrently exist at each base surface of regrowth, the Asxe2x80x94P interdiffusion can be suppressed, for example, when the temperature is raised in a phosphorus atmosphere before the the second conductive type InGaP second upper cladding layer and the second conductive type GaAs contact layer are formed. Therefore, it is possible to improve the quality of the regrown crystal.
(v) Further, since the double-layer etching stop layers constituted by the InGaP layer and the GaAs layer are used, the precision in etching can be improved, i.e., the distribution of the equivalent refractive index which realizes the ARROW structure can be formed with high precision.
(vi) In the case where the first current confinement layer is constituted by first and second sublayers made of GaAs of the first conductive type and a quantum-well layer formed between the first and second sublayers and made of an InGaAs material which has a smaller bandgap than the bandgap of the compressive-strain quantum-well active layer, the gain of oscillation in the fundamental transverse mode can be increased since the InGaAs quantum-well layer absorbs light.
(vii) In the case where a second lower cladding layer made of AlxGa1-xAs (0.45xe2x89xa6xxe2x89xa60.55) of the first conductive type is formed between the GaAs substrate and the first lower cladding layer, leakage of carriers from the active region can be effectively suppressed since AlGaAs has a greater bandgap than InGaP.
Although the first lower cladding layer per se may be formed with only AlxGa1-xAs, the provision of both of the InGaP first lower cladding layer and the AlGaAs second lower cladding layer is more advantageous because it is possible to reduce the time needed for changing gas before the InGaAsP lower optical waveguide layer is formed, and improve the quality of the interface between the InGaAsP lower optical waveguide layer and the InGaP first lower cladding layer.
Further, when the thickness of the InGaP first lower cladding layer is reduced to 0.5 micrometers or smaller, the semiconductor layers can be formed without deterioration of surface morphology.